1. Field of the Invention
The present invention relates to a highly selective method for the deacetylation and deacylation of taxol and taxanes compounds. Specifically, the present invention relates to a one step process wherein acyl groups located at the carbon 2', 10, and 7 positions of taxol and other taxane compounds may be selectively removed.
2. Description of the State of Art
Between the years 1958 and 1980, extracts of over 35,000 plant species were tested for anticancer activity as part of an NCI-sponsored program. Chemists Monroe E. Wall and M. C. Wani first isolated a crude extract concentrate from yew tree (Taxus brevifolia) bark and wood samples in 1963. Initial screening showed the extract to be a potential anticancer agent, being very active against an unusually wide range of rodent cancers. Isolation of the active agent in the crude extract took several years due to the very low concentrations of the agent present in the plants. The active agent was identified, the structure determined and the compound was named taxol, in 1971. ##STR1## Despite taxol's excellent activity in model tumor systems, clinical trails were delayed owing to short supplies of the drug and formulation problems related to the drug's low water-solubility. However, great interest in the drug was rekindled when it was discovered in 1979 by Susan B. Horwitz and co-workers that a unique mechanism for taxol's antitumor activity involved cell microtubules. See, Nature 277: 665-667 (1979). Microtubules play a key role in mitosis, maintenance of cell shape, cell motility, and intracellular transport. They are self-assembling and self-disassembling structures that are in dynamic equilibrium with tubulin dimers, the protein subunits of which they are composed. A substance that interferes with microtubules can disrupt cell growth and function.
The 1979 study by S. Horwitz et al., reported that the binding of taxol to tubulin acts to stabilize cell microtubules and to prevent their depolymerization. Thus, taxol increases the time required for cell division which in turn inhibits tumor activity. Discovery of this unique mechanism, by which taxol disrupts the proliferation of cancerous cells, intensified research interest in the drug, and the National Cancer Institute (NCI) began a concerted effort to obtain taxol for clinical trials. In ongoing clinical trials, taxol has shown promising results in fighting advanced cases of ovarian, breast, and other cancers.
In spite of taxols excellent activity, its development as a clinical agent has been impeded both by the difficulty of isolation from the bark of the yew and its poor water-solubility. One approach to overcome the water-solubility drawback while possibly increasing potency is to develop various analogues of taxol. Taxotere is a new semisynthetic taxol analogue prepared at the Institut de Chimie des Substances Naturelie of the Centre National de la Recherche Scientifique (Gif sir Yvette, France). Taxotere was obtained through partial synthesis using a precursor 10-deacetylbaccatin III.
In a technical paper by Kingston et al., entitled "Modified Taxols. 5. Reaction of Taxol with Electrophilic Reagents and Preparation of a Rearranged Taxol Derivative with Tubulin Assembly Activity", J. Org. Chem. 56: 5114-5119 (1991) it was reported that taxol when treated under the mildest conditions of zinc bromide in methanol at ambient temperatures produces 10-deacetyltaxol (10-DAT) along with 10-deacetyl-7-epitaxol (7-epi-10-DAT). Selective deacetylation procedures were also reported by Chen et al, in "The Chemistry of Taxanes: Reaction of Taxol and Baccatin Derivatives With Lewis Acids in Aprotic and Protic Media," Tetrahedron 49(14): 2805-2828 (1993). Chen et al. reported performing selective deacetylation using Lewis acids. All zinc halides used gave results analogous to the ones observed with zinc bromide. The same was hue for many salts tried, such as Ce (III) and Mg (II). Other Lewis acids (notably CsF and LiI) were found to cleave the side chain of taxol.
While it is possible to extract 10-deacetylbaccatin III (10-DAB) from the needles of the European Yew, Taxus baccata, it would be desirable to have a process that would convert other taxane compounds into 10-deacetylbaccatin III. While both Kingston et al. and Chen et al. report methods for the deacetylation of taxol resulting in compounds having greater water-solubility and new functional groups both methods suffer from the inability to entirely suppress the epimerization reaction at the C-7 position, thus resulting in low yields of 10-deacetyltaxol. A further disadvantage of the Lewis acids used by Chen et al. is that cleavage of the side chain readily occurs.
There is still a need, therefore, for a method for the selective deacylation including deacetylation of taxol and other taxanes so that various analogues of taxol having improved water-solubility and potency can be developed.